Ferroelectric liquid crystal display and fabricating method thereof

ABSTRACT

A liquid crystal display cell, and a fabricating method thereof, that incorporates a light-cured monomer in a ferroelectric liquid crystal. By injecting the light-cured monomer/ferroelectric liquid crystal between substrate while in a nematic phase or isotropic phase, by performing DC voltage treatments as the liquid crystal temperature decreases, and by performing a light irradiation treatment, a stable, aligned ferroelectric liquid crystal cell is produced. Such a ferroelectric liquid crystal cell can reduce flicker and can be realigned after a physical shock.

CROSS REFERENCE

[0001] This application claims the benefit of Korean Patent ApplicationNo. P2000-86963, filed 30 Dec. 2000, under 35 U.S.C. §119, the entiretyof which is hereby incorporated by reference.

BACKGROUND OF THE INVENTION

[0002] 1. Field of the Invention

[0003] This invention relates to a liquid crystal display, and moreparticularly to a ferroelectric liquid crystal display having reducedflicker and a stable liquid crystal alignment.

[0004] 2. Description of the Related Art

[0005] A liquid crystal display (LCD) controls the light characteristicsof a display screen so as to produce a desired image. Liquid crystalsused in liquid crystal displays are in a neutral state between a liquidand a solid. That neutral state has both fluidity and elasticity.

[0006] While there are many types of liquid crystals, one type of greatinterest is the smectic C liquid crystal. During a thermodynamic phasetransition, smectic C liquid crystal molecules rotate along an outerline of a virtual cone. Such a smectic C phase liquid crystal canundergo a spontaneous polarization. Such a liquid crystal is usuallyreferred to as a “ferroelectric liquid crystal” (FLC). The FLC has beenactively studied because of its fast response time. Furthermore, FLCLCDs can have wide viewing angles without the complications of specialelectrode structures or compensating films.

[0007] There are many different FLC modes, including a deformed helixFLC, a surface stabilized FLC, an anti-FLC, a V-mode FLC and a halfV-mode FLC. Hereinafter, the V-mode FLC mode and the half V-mode FLCmode will be described in more detail.

[0008]FIG. 1 shows a V-mode FLC liquid crystal cell. As shown, thatliquid crystal cell includes an upper substrate 1 having a commonelectrode 3 and an alignment film 5. That liquid crystal cell alsoincludes a lower substrate 11 having a TFT array 9, which includes pixelelectrodes, and an alignment film 7. A V-mode liquid crystal 13 isinterposed between the upper and lower substrates 1 and 11. Thealignment films 5 and 7 are aligned in a horizontal direction, usuallyby rubbing the alignment layers with a special cloth roller. The V-modeliquid crystal 13 forms multiple smectic layers that have molecularstructures arranged with desired slopes with respect to a planeperpendicular to the smectic layers. In other words, the liquid crystalmolecules have desired inclination angles with respect to the horizontalalignment direction of the alignment films. Furthermore, adjacentsmectic layers have opposite polarities.

[0009] Light transmission through a V-mode FLC liquid crystal cellvaries according to an applied voltage across that cell, reference FIG.2. The liquid crystal 13 within the V-mode FLC liquid crystal cellresponds to both positive and negative voltages. Since the lighttransmissivity rapidly changes in accord with applied positive andnegative voltages, the light transmissivity verses voltage curve has theV shape shown in FIG. 2. Thus, light transmissivity increases regardlessof polarity.

[0010]FIG. 3 shows an alignment state of a half V-mode FLC liquidcrystal cell. As shown, a half V-mode FLC liquid crystal 15 isinterposed between an upper substrate 1 and a lower substrate 11. Thehalf V-mode FLC liquid crystal 15 forms multiple smectic layers in whichthe liquid crystal molecules align at desired inclination angles withrespect to a horizontal alignment direction of the alignment films 5 and7. However, as shown in FIG. 3, the liquid crystal molecules in adjacentsmectic layers have the same polarity (unlike V-mode FLC liquid crystalmolecules). Such a half V-mode FLC liquid crystal can be formed byapplying a positive (or a negative) electric field across a hot liquidcrystal, and, at the same time, lowering that liquid crystal'stemperature into a smectic phase.

[0011] A half V-mode FLC mode liquid crystal 15 formed in this mannerresponds to only one polarity of applied voltage. Thus, as shown in FIG.4, a light transmissivity verse voltage curve of a half V-mode FLCliquid crystal cell has a ‘half V’ shape. Still referring to FIG. 4, asshown, the light transmissivity verses voltage curve does react,slightly, to negative applied voltages, but dramatically to positiveapplied voltages.

[0012] The light transmissivity curve shown in FIG. 4 represents aliquid crystal cell in which the liquid crystal molecules are aligned bya negative voltage. In this case, the light transmissivity of the liquidcrystal cell almost does not increase when a negative voltage isapplied, but rapidly increases when a positive voltage is applied. Onthe other hand, a liquid crystal aligned by a positive voltage increasesits light transmissivity with an increase in a negative voltage.

[0013] The thermodynamic phase transition of a half V-mode FLC liquidcrystal 15 is as follows:

Isotropic→nematic (N*) phase→smectic C* (Sm C*) phase→crystal

[0014] Such thermodynamic phase transitions express the phases of theliquid crystal in accordance with temperature, which becomes less asphase changes move to the right.

[0015] An isotropic phase liquid crystal 15 interposed into a liquidcrystal cell aligns in parallel with the rubbing direction of analignment layer when the liquid crystal temperature is slowly lowered tothe nematic phase. If a sufficiently strong electric field is appliedacross the liquid crystal cell while the liquid crystal temperature isslowly lowered, the liquid crystal 15 is phase-changed into a smecticphase in which the direction of spontaneous polarization of the liquidcrystal molecules is in accord with the electric field. Consequently,when the liquid crystal 15 within the liquid crystal cell is subjectedto a parallel alignment treatment, the liquid crystal molecules arrangein a spontaneous polarization direction that is consistent with theelectric field at the phase transition, and in one of two possiblemolecular arrangements. As a result, the liquid crystal 15 has a uniformalignment state.

[0016]FIG. 5 and FIG. 6 help illustrate this. First, as shown in FIG. 5,if a negative electric field E(−) is applied during alignment of theliquid crystal 15, then the spontaneous polarization direction of theliquid crystal 15 is along the electric field. In such an aligned liquidcrystal cell, as shown in FIG. 6, the liquid crystal arrangement ischanged by an applied positive electric field E(+), while a liquidcrystal arrangement is not changed by an applied negative electric fieldE(−).

[0017] To utilize the response characteristics of the liquid crystal 15,perpendicular polarizers are arranged on the upper and lower portions ofthe liquid crystal cell. The transmission axis of one of the polarizersis along the direction of the initial liquid crystal alignment. Assuminga liquid crystal cell having the transmission curve of FIG. 4, anapplied negative electric field E(−) does not change the liquid crystalarrangement and the perpendicular polarizer blocks light. A positiveelectric field E(+) rotates the liquid crystal alignment such that lighttransmission increases.

[0018] As described above, the half V-mode FLC liquid crystal cell usesboth temperature and an applied electric field during alignment.However, such a liquid crystal cell has a problem in that breaking theinitial alignment, such as by external impacts that are inevitablyapplied by grinding a shorting bar, destroys that alignment. Tore-establish alignment, both temperature and electric fields must beused. However, this is difficult to do once the shorting bar, which wasused for applying the electric fields, is removed. In addition to shock,heating a conventional half V-mode FLC liquid crystal cell can destroythe alignment.

[0019] Furthermore, the conventional half V-mode FLC liquid crystal cellhas a 30 Hz flicker in which light is transmitted at 30 Hz with respectto an alternating current (AC) driving signal of 60 Hz, while light isnot transmitted with respect to the remaining driving signal.Accordingly, it is necessary to address the flicker problem to image astationary picture with an acceptable gray level.

[0020] One approach to flicker is to divide the pixel area into twosections having contrary electric charges. However, this scheme iscomplex to implement, particularly with high brightness.

[0021] An alternative scheme for addressing the flicker problem includesincreasing the driving signal frequency. In other words, it may bepossible to obtain a 60 Hz transmission characteristic (in comparison tothe 30 Hz) by driving the display at 120 Hz (instead of at 60 Hz).However, this requires development of a novel driver IC, and thusincreases cost.

[0022] Another approach to reducing flicker is to use the FLC liquidcrystal cell shown in FIG. 7. Referring now to FIG. 7, that FLC liquidcrystal cell has the uniform alignment characteristics of a half V-modeFLC liquid crystal cell. Also, the illustrated FLC liquid crystal cellhas the symmetrical driving characteristics of the V-mode FLC liquidcrystal cell, reference FIG. 8. The illustrated FLC liquid crystal cellhas symmetrical driving characteristics because the liquid crystal ispositioned at a central portion of a virtual cone area, which representsthe rotatable positions, in accordance with temperament or rubbing ofthe alignment film. The illustrated liquid crystal cell enables theprimary alignment direction of the liquid crystals to be consistent withthe rubbing treatment of the alignment films.

[0023] However, the FLC liquid crystal cell illustrated in FIG. 7 ishighly sensitive to the process condition of the alignment film. Sincethe FLC liquid crystal cell illustrated in FIG. 7 has a small alignmenttolerance, the temperament conditions and the rubbing of the; alignmentfilm are critical. As a result, it becomes difficult to achieve equalalignment forces on the upper alignment film and on the lower alignmentfilm. Furthermore, because of problems with achieving uniform alignment,the FLC liquid crystal cell illustrated in FIG. 7 has proven difficultto mass produce, particularly in large dimension LCDs.

SUMMARY OF THE INVENTION

[0024] Accordingly, it is an object of the present invention to providea ferroelectric liquid crystal display, and a fabricating methodthereof, with reduced flicker. It is another object of the presentinvention to provide a ferroelectric liquid crystal display withimproved liquid crystal alignment stability.

[0025] To achieve these and other objects of the invention, aferroelectric liquid crystal display according to one aspect of thepresent invention includes: an upper substrate and a lower substrate,each having an alignment film, and a ferroelectric liquid crystalinterposed between the upper substrate and the lower substrate. Thatferroelectric liquid crystal includes an additive for forming a stablepolymer network. Beneficially, the ferroelectric liquid crystal displayhas alignment films on the upper and lower substrates to have parallelalignment treating directions. Even more beneficially, the additive islight sensitive.

[0026] A method of fabricating a ferroelectric liquid crystal displayaccording to another aspect of the present invention includes providingan upper substrate and a lower substrate; forming alignment films on theupper and lower substrates; joining the upper substrate with the lowersubstrate; interposing a ferroelectric liquid crystal having alight-curing monomer between the upper and lower substrates; applying aninitial DC voltage to uniformly align the liquid crystal; applying a DCvoltage of the opposite polarity (to that of the initial DC voltage) toorientate the liquid crystal with the alignment films; and irradiatinglight onto the light-curing monomer to form a polymer network.

[0027] In the method, the temperature of the interposed liquid crystalis above the phase-change temperature of the smectic phase (being in anisotropic or nematic phase). The method further includes raising thetemperature of the liquid crystal cell to change the liquid crystal intoa nematic phase after curing the polymer; and lowering a temperature ofthe nematic phase liquid crystal to change the liquid crystal into asmectic phase.

BRIEF DESCRIPTION OF THE DRAWINGS

[0028] These and other objects of the invention will be apparent fromthe following detailed description of the embodiments of the presentinvention with reference to the accompanying drawings, in which:

[0029]FIG. 1 illustrates an alignment state of a conventional V-mode FLCliquid crystal cell;

[0030]FIG. 2 is a graph representing light transmissivity verses voltageof the liquid crystal cell illustrated in FIG. 1;

[0031]FIG. 3 illustrates an alignment state of a conventional halfV-mode FLC liquid crystal cell;

[0032]FIG. 4 is a graph representing light transmissivity verses voltageof the liquid crystal cell illustrated in FIG. 3;

[0033]FIG. 5 illustrates applying an electric field to a half V-mode FLCliquid crystal cell;

[0034]FIG. 6 depicts the motion of liquid crystal molecules in a halfV-mode FLC liquid crystal cell;

[0035]FIG. 7 illustrates an alignment state of a liquid crystal cell inan alternative mode liquid crystal cell;

[0036]FIG. 8 is a graph representing light transmissivity verses voltageof the FLC liquid crystal cell illustrated in FIG. 7;

[0037]FIG. 9A to FIG. 9C illustrate an alignment process of an FLCliquid crystal cell according to an embodiment of the present invention;and

[0038]FIG. 10 is a flow chart of the aligrnent process illustrated inFIGS. 9A to 9C.

DETAILED DESCRIPTION OF THE ILLUSTRATED EMBODIMENT

[0039]FIGS. 9A to 9C and FIG. 10 illustrate fabricating an FLC liquidcrystal cell according to an embodiment of the present invention. InFIG. 9A, the FLC mode liquid crystal cell includes an upper substrate 21having a color filter layer (not shown), a common electrode 23, and analignment film 25 that are sequentially disposed. That FLC liquidcrystal cell also includes a lower substrate 31 having a thin filmtransistor (TFT) array 29 and an alignment 27. The thin film transistor(TFT) array 29 includes a plurality of pixel electrodes. After the uppersubstrate 21 and the lower substrate 31 are joined, a ferroelectricliquid crystal 33, which contains a small amount of a light-curedmonomer 35, is injected between the upper substrate 21 and the lowersubstrate 31. Beneficially, the ferroelectric liquid crystal 33 isinjected in the dark.

[0040] Referring now to FIG. 10, in a step SI the ferroelectric liquidcrystal 33 is maintained during injection at a temperature at which theliquid crystal has an isotropic phase or a nematic phase. Injection atsuch a temperature prevents damage to arranged liquid crystals, whichcould form due to the low injection rate of the liquid crystal, and dueto shear stress to the alignment films. The low injection rate and theshear stress result from that the viscosity of a smectic phase liquidcrystal being much larger than that of a nematic phase liquid crystal.

[0041] In step S2, a DC voltage is applied across the upper and lowersubstrates 21 and 31 while slowly lowering the temperature of the liquidcrystal cell. This causes the liquid crystal 33 to uniformly align inthe direction of a virtual cone, which indicates a rotatable path.Accordingly, the liquid crystal 33 is changed into a smectic phase and,at the same time, is initially aligned into a particular direction. Thisforms a mono-domain initial alignment.

[0042] Referring now to FIG. 10 and to FIG. 9B, a DC voltage having apolarity contrary to the DC voltage that was used for the initialalignment is then applied, step S3. This arranges the liquid crystals 33at the center of the cone, from which the liquid crystal 33 can be movedby applied potentials. This position is beneficially parallel to thealignment treatment direction. In this case, the DC voltage thatpositions the liquid crystal 33 at the center of the cone is dependenton the light-cured monomer 35 and on the spontaneous polarization of theliquid crystal 33.

[0043] Referring now to FIG. 10 and to FIG. 9C, at step S4, theuniformly aligned liquid crystal from step S4 is exposed to ultravioletlight. This cures the light-cured monomer 35 into a network.Accordingly, the uniformly aligned liquid crystal 33 becomes stable.Beneficially, such a liquid crystal 33 can be driven by both positiveand negative electric fields.

[0044] Referring now to FIG. 10, in step S5 the liquid crystal cell isheat treated by raising its temperature to a nematic phase. Thisstabilizes the liquid crystal alignment. The liquid crystal celltemperature is then slowly lowered to form a smectic phase. In thiscase, the liquid crystal 33 is stabilized by a polymer network structureto position liquid crystal molecules at the center of the virtual cone.

[0045] An FLC liquid crystal cell according to the principles of thepresent invention that has its liquid crystal alignment broken by anexternal impact can be re-aligned by a heat treatment. Also, themolecule arrangement and the driving characteristics of an FLC accordingto the present invention are similar to those of the FLC liquid crystalcell illustrated in FIG. 7. Furthermore, an FLC liquid crystal cellaccording to the principles of the present invention can be driven bypositive and negative voltages such that flicker can be reduced.Moreover, an FLC liquid crystal cell according to the principles of thepresent invention can have an improved rotation angle and improved lighttransmissivity.

[0046] As described above, according to the present invention, theliquid crystal is positioned at the center of an FLC rotation cone and apolymer network is formed, thereby assuring alignment stability.Accordingly, it becomes possible to provide a stable liquid crystalalignment. By providing a liquid crystal centered in the cone it becomespossible to drive the liquid crystal cell with either polarity, whichenables reduced flicker, thereby improving picture quality.

[0047] Although the present invention has been explained by theembodiments shown in the drawings described above, it should beunderstood to the ordinary skilled person in the art that the inventionis not limited to the embodiments, but rather that various changes ormodifications thereof are possible without departing from the spirit ofthe invention. Accordingly, the scope of the invention shall bedetermined only by the appended claims and their equivalents.

What is claimed is:
 1. A ferroelectric liquid crystal display,comprising: an upper substrate having a first alignment film; a lowersubstrate having a second alignment film, wherein said lower substrateis attached to said upper substrate such that said first alignment filmand said second alignment film face; and a ferroelectric liquid crystalinterposed between said upper substrate and said lower substrate,wherein said ferroelectric liquid crystal includes a polymer network. 2.The ferroelectric liquid crystal display according to claim 1, whereinsaid first alignment film and said second alignment film have parallelalignment directions.
 3. The ferroelectric liquid crystal displayaccording to claim 1, wherein said ferroelectric liquid crystal isaligned in parallel with an alignment direction of said first alignmentfilm.
 4. The ferroelectric liquid crystal display according to claim 1,wherein said upper substrate includes a color filter layer.
 5. Theferroelectric liquid crystal display according to claim 4, wherein saidupper substrate includes a common electrode.
 6. The ferroelectric liquidcrystal display according to claim 1, wherein said lower substrateincludes a pixel electrode.
 7. The ferroelectric liquid crystal displayaccording to claim 1, wherein said ferroelectric liquid crystal ispositioned at a central rotation angle of a virtual cone.
 8. Theferroelectric liquid crystal display according to claim 1, wherein saidpolymer network is formed from a light-curing monomer.
 9. A method offabricating a ferroelectric liquid crystal display, comprising:providing an upper substrate and a lower substrate; forming alignmentfilms on the upper and lower substrates; joining the upper substratewith the lower substrate; interposing a ferroelectric liquid crystalhaving a light-curing monomer between the joined upper and lowersubstrates; applying an initial DC voltage across the liquid crystal;subsequently applying a second DC voltage across the liquid crystal,wherein said second DC voltage has a polarity opposite that of theinitial DC voltage; and irradiating light onto the ferroelectric liquidcrystal such that the light-curing monomer forms a polymer network. 10.The method according to claim 9, further comprising: raising atemperature of the ferroelectric liquid crystal to force theferroelectric liquid crystal into a nematic phase after forming thepolymer network; and lowering a temperature of the nematic phaseferroelectric liquid crystal to force the ferroelectric liquid crystalinto a smectic phase.
 11. The method according to claim 9, wherein theferroelectric liquid crystal is interposed at a temperature above thesmectic phase-to-nematic phase transition temperature.
 12. The methodaccording to claim 9, wherein the alignment direction of the alignmentfilm on the upper substrate is parallel with the alignment direction ofthe alignment film on the lower substrate.
 13. The method according toclaim 12, wherein the ferroelectric liquid crystal is aligned with thealignment direction of the alignment film on the upper substrate. 14.The method according to claim 9, fuirther including forming a colorfilter layer and a common electrode on the upper substrate.
 15. Themethod according to claim 9, further including forming a TFT array layerhaving a pixel electrode on the lower substrate.
 16. The methodaccording to claim 9, wherein irradiating light onto the ferroelectricliquid crystal includes irradiating with ultra-violet light.
 17. Themethod according to claim 9, wherein interposing a ferroelectric liquidcrystal with a light-curing monomer between the joined upper and lowersubstrates is performed in the dark.